Size reduction

Size reduction of foods through various mills hammer mill, burr mill, ball mill, energy consumption in size reduction

1. SIZE REDUCTION FOOD PRODUCTS DEVINDER DHINGRA

2. Size reduction of solids – converting large particles into smaller/fine particles of the same material. Size reduction is brought by application of force on larger particles. Types of forces applied: (i) Compression (ii) Impact, and (iii) Shear Size reduction process is also known as comminution or grinding. Reduction in particle size by mechanical means is known as milling. SIZE REDUCTION

3. Classification of Size Reduction Chopping, Cutting, Slicing and Dicing Slicing / Cutting of fruits / vegetables (canning, minimal processing, value addition-chips) Mincing of meat Dicing of carrots Shredding of vegetables Milling into powders /pastes Spices Flour Smooth pastes (peanut butter) Homogenisation Milk (reduction in particle size of fat globules)

4. Compressive forces are generally used for the coarse crushing of hard materials. Careful application of compressive forces enables control to be exercised over the breakdown of the material, e.g. to crack open grains of wheat to facilitate separation of the endosperm from the Bran. Impact forces are used to mill a wide variety of materials, including fibrous foods. Shear forces are best applied to relatively soft materials, again including fibrous foods. All three types of force are generated in most types of mills, but generally one predominates. For example, in most roller mills compression is the dominant force, impact forces feature strongly in hammer mills and shear forces are dominant in disc attrition

5. Roller Mill Roller surface could be plain or serrated depending on the requirement Wheat Milling in a roller flour mill is done by such machines and wheat grains are converted into flour through number of stages of roller mills

6. Hammer Mill Predominant Force: Impact Commonly used for spices: turmeric, pepper, chillies etc.

7. Disc Mill Interacting surfaces of discs are rough/corrugated The ground particles leave the edge of the discs Marge particles are fed at the centre The discs could be vertical or horizontal Example: Atta Chakkis Predominant force: Shear (& compression)

8. Reduction Ratio The extent of the reduction in size of the feed particles/raw material can be expressed by the reduction ratio The term average size depends on the method of measurement. In the food industry, screening or sieving is widely used to determine particle size distribution in granular materials and powders. In this case, the average diameter of the particles is related to the aperture sizes of the screens used. Size reduction ratios vary from below 8:1 in coarse crushing to more than 100:1 in fine grinding.  

9. Why Reduction in Size Size reduction may aid the extraction of a desired constituent from a composite structure, e.g. flour from wheat grains or juice from sugar cane. Reduction to a definite size range may be a specific product requirement; e.g. as in the manufacture of icing sugar, in the preparation of spices and in chocolate refining. A decrease in particle size of a given mass of material leads to an increase in surface of the solid. This increase in surface is of assistance in many rate process, e.g. The drying time for moist solids is much reduced by increasing the surface area of the solid. The rate of extraction of a desired solute is increased by increasing the contact area between solid and solvent. Process time required for certain operation such as cooking, blanching, etc.-can be reduced by cutting, shredding or dicing the process material. Thorough mixing or blending is usually easier with small size ranges of particles, an important consideration in the production of formulated packaged soups, cake mixes, etc.

10. Size reduction It is desirable to produce particles in the specified size range (to meet the standards/requirements of further processing operations etc.) Screens are used to obtain particles in the specified size range. Multiple stages / machines and screens may be used in size reduction, depending on the raw material and finished product.

11. Application of Force causes strain

12. When a solid material is subjected to a force, its behavior may be represented by a plot of stress versus strain, Some materials exhibit elastic deformation when the force is first applied. The strain is linearly related to stress (see curve 2 in Fig.43.1). If the force is removed the solid object returns to its original shape. Elastic deformations are valueless in size reduction. Energy is used up but no breakdown occurs. Point E is known as the elastic limit. Beyond this point, the material undergoes permanent deformation until it reaches its yield point Y . Brittle materials will rupture at this point. Ductile materials will continue to deform, or flow, beyond point Y until they reach the break point B , when they rupture. The behaviour of different types of material is depicted by the five curves in Fig 43.1. and explained in the legend on the figure. The breakdown of friable materials may occur in two stages. Initial fracture may occur along existing fissures or cleavage planes in the body of the material. In the second stage new fissures or crack tips are formed and fracture occurs along these fissures. Larger particles will contain more fissures than smaller ones and hence will fracture more easily. In the case of small particles, new crack tips may need to be created during the milling operation. Thus, the breaking strength of smaller particles is higher than the larger ones. The energy required for particle breakdown increases with decrease in the size of the particles. In the limit of very fine particles, only intermolecular forces must be overcome and further size reduction is very difficult to achieve. However, such very fine grinding is seldom required in food applications. Only a very small proportion of the energy supplied to a size reduction plant is used in creating new surfaces. Literature values range from 2.0% down to less than 0.1%. Most of the energy is used up by elastic and inelastic deformation of the particles, elastic distortion of the equipment, friction between particles and between particles and the equipment, friction losses in the equipment and the heat, noise and vibration generated by the equipment.

13. Mathematical Equations for Energy Analysis in size reduction The relationship between the comminution energy and the product size obtained for a given feed size has been researched extensively over the last century. Theoretical and empirical energy-size reduction equations were proposed by Rittinger (1867), Kick (1885) and Bond (1952), known as the three theories of comminution ; and their general formulation by Walker et al. (1937) …………………….(1) Where E is the net specific energy; x is the characteristic dimension of the product; n is the exponent ; and C is a constant related to the material. Equation [1] states that the required energy for a differential decrease in size is proportional to the size change (dx) and inversely proportional to the size to some power n. If the exponent n in Equation [1] is replaced by the values of 2, 1 and 1.5 and then integrated, the well-known equations of Rittinger , Kick and Bond, are obtained respectively  

14. Rittinger (1867) stated that the energy required for size reduction is proportional to the new surface area generated. Since the specific surface area is inversely proportional to the particle size, Rittinger’s hypothesis can be written in the following form: where E is energy per unit mass required, K1 is constant and x p and x f are avg particle size of product and feed. Observed to be good for fine grinding  

15. Kick (1885) proposed the theory that the equivalent relative reductions in sizes require equal energy. Kick’s equation is as follows : where E is energy per unit mass required, K 2 is constant and x p and x f are avg particle size of product and feed Observed to be good for coarse grinding  

16. Bond (1952) proposed the ‘Third Law’ of grinding. The Third Law states that the net energy required in comminution is proportional to the total length of the new cracks formed . The resulting equation is: where E is energy per unit mass required, K 3 is constant and x p and x f are avg particle size of product and feed Only a very small proportion of the energy supplied to a size reduction plant is used in creating new surfaces. Literature values range from 2.0% down to less than 0.1%. Most of the energy is used up by elastic and inelastic deformation of the particles, elastic distortion of the equipment, friction between particles and between particles and the equipment, friction losses in the equipment and the heat, noise and vibration generated by the equipment.  

17. SIZE REDUCTION EQUIPMENT

18. SIZE REDUCTION EQUIPMENT a) Roller mill A common type of roller mill consists of two cylindrical steel rolls, mounted on horizontal axes and rotating towards each other. The particles of feed are directed between the rollers from above. They are nipped and pulled through the rolls where they are subjected to compressive forces, which bring about their breakdown. If the rolls turn at different speeds shear forces may be generated which will also contribute to the breakdown of the feed particles .

19. Flour Roller Mill Roller surface SIZE REDUCTION EQUIPMENT

20. Hammer mill SIZE REDUCTION EQUIPMENT

21. Ball Mill In the ball mill both shearing and impact forces are utilized in the size reduction. The unit consists of a horizontal, slow speed-rotating cylinder containing a charge of steel balls or flint stones. As the cylinder rotates balls are lifted up the sides of the cylinder and drop on to the material being comminuted, which fills the void spaces between the balls. The balls also tumble over each other, exerting a shearing action on the feed material. This combination of impact and shearing forces brings about a very effective size reduction. Ball sizes are usually in the range 1 – 6 inches. Small balls give more point contacts but larger balls give greater impact. As with all grinding mills, working surfaces gradually wear, so product contamination must be guarded against . SIZE REDUCTION EQUIPMENT

22. At low speeds of rotation the balls are not lifted very far up the walls of the cylinder. The balls tumble over each other and shear forces predominate. At faster speeds the balls are lifted further and the impact forces increase. Attrition and impact forces play a part in reduction. At high speeds the balls can be carried round at the wall of the mill under the influence of centrifugal force. Under these conditions grinding ceases. For efficient milling the critical speed should not be exceeded. This is defined as the speed at which a small sphere inside the mill just begins to centrifuge. The critical speed is the speed at which centrifugal forces equal gravitatinal forces at the inside surface of the shell of ball mill and no balls will fall from its position to the shell. Effective grinding will not happen. It can be shown that the critical speed N c in r.p.m ., is given by: (derivation on next slide) Where, R is the radius of the mill, in m In practice, the optimum operating speed is about 75% of the critical speed and should be determined under the plant operating conditions.  

23. Critical speed ϴ R mg mV 2 /(R-r) R = radius of ball mill shell R=radius of ball N= rotational speed of ball mill shell (revolutions per second) m = mass of ball R-r = effective radius for balancing forces V=linear peripheral velocity=2 π ( R-r)N g= Acceleration due to gravity m/min ϴ Critical Speed Centrifugal force = gravitational force mV 2 /(R-r) cos ϴ = mg At ϴ =0, the ball will be at the top and will not fall down at critical speed (N= Nc ) Solving above equation yields Critical speed (rpm) = N c *60  

24. Ball Mill

25. Pin Mill In one type of pin mill a stationary disc and rotating disc are located facing each other, separated by a small clearance. Both discs have concentric rows of pins, pegs or teeth. The rows of one disc fit alternately into the rows of the other disc. The pins may be of different shapes; round, square or in the form of blades. The feed in introduced through the centre of the stationary disc and passes radially outwards through the mill where it is subjected to impact and shear forces between the stationary and rotating pins. The mill may be operated in a choke feed mode by having a screen fitted over the whole or part of the periphery. SIZE REDUCTION EQUIPMENT

26. Attrition mills SIZE REDUCTION EQUIPMENT

27. Mechanical Properties of the Feed Moisture Content of the Feed Temperature Sensitivity of the Feed Selection Criteria of Size Reduction Equipment

28. Size reduction of fat globules Homogenizers use a high pressure pump, operating at 100–700 bar, Which is fitted with a homogenizing valve(s) (two-stage homogenization) on the discharge side. When liquid is pumped through the small adjustable gap ( < 300 μm ) between the valve and the valve seat. The high pressure produces a high liquid velocity (80–150 m/s).

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